(1) Field of the Invention
The present invention relates to a process for preparing titanium and a titanium alloy material having a superior fatigue strength and workability particularly a process for preparing a titanium, .alpha. titanium alloy or (.alpha.+.beta.) titanium alloy having a fine equiaxed microstructure.
(2) Description of the Related Art
Titanium and its alloys have been used in various material applications, including aerospace materials, owing to their high strength-to-density ratio and high corrosion resistance, and the applications thereof are expanding. The reason why titanium and .alpha. and (.alpha.+.beta.) titanium alloys are in such great demand is that they have a high strength and ductility, but the characteristics requirements in each field are very strict, and in particular, aerospace materials, etc., used under an environment subject to cyclic stresses must have superior fatigue properties, in addition to a good workability. This has led to establishment of strict quality standards (e.g., as seen in AMS4967), and to meet such requirements, the .alpha. grain of the material must have a fine equiaxed microstructure.
Since the impurity contents of titanium are limited, an equiaxed microstructure can be obtained by the conventional working and heat treatment, but it has been difficult to homogeneously refine the microstructure.
On the other hand, products used in the above-described field and having various shapes (plate, wire, tube, rod, etc.) and made of .alpha. and (.alpha.+.beta.) titanium alloys, are usually manufactured by a combination of hot working and heat treatments. The step of the hot working, however, has a drawback that a proper working temperature range is too narrow to satisfy both of the following requirements; (1) ensuring of a good workability for attaining a very precise product shape, (2) a formation of an equiaxed microstructure in the product.
Further, in the above-described temperature range, the microstructure is highly sensitive to temperature change; for example, even a slight rise in the temperature causes grain growth, and thus the microstructure after working tends to become heterogeneous. Further, the microstructure formed during hot working does not undergo any significant change.
This has led to proposals for a process for preparing .alpha. and (.alpha.+.beta.) titanium alloys having an equiaxed microstructure, e.g., a preparation process disclosed in Japanese Examined Patent Publication No. 63-4914 wherein heating and working are repeated in a specific narrow temperature range, and a preparation process disclosed in Japanese Examined Patent Publication No. 63-4908, wherein a hot rolling material is heated at temperatures above the .beta. transformation point. Nevertheless, these processes cannot satisfactorily attain a homogeneously fine equiaxed microstructure of a material. Further, the former is disadvantageous in that the productivity is poor and the production cost is high.
Techniques which utilize hydrogen as a temporary alloying element in titanium alloys for improving their workability and microstructure are disclosed in the following literature.
(1) U. Zwicker et al., U.S. Pat. No. 2,892,742 (issued on Jun. 30, 1959): PA1 (2) W.R. Kerr et al., "Hydrogen as an alloying element in titanium (Hydrovac)", Titanium ,80, P. 2477-2486: PA1 (3) N. C. Birla et al., "Anisotropy control through the use of hydrogen in Ti-6Al-4V alloy", Transactions of the Indian Institute of Metals, Vol. 37, No. 5, Oct. 1984, P. 631-635: PA1 (4) D. Eylon et al., U.S. Pat. No. 4,820,360 (Apr. 11, 1989): PA1 (5) D. Eylon et al., U.S. Pat. No. 4,832,760 (May 23, 1989): PA1 (6) W. R. Kerr, "The Effect of Hydrogen as a Temporary Alloying Element on the Microstructure and Tensile Properties of Ti-6Al-4V", METALLURGICAL TRANSACTIONS A, Vol. 16A, June 1985, P. 1077-1087:
This patent describes that an .alpha. titanium alloy having an Al content of 6% or more is hydrogenated in an amount of 0.05 to 1.0% by weight of hydrogen, to improve the hot workability, and finally, dehydrogenated in vacuum, but makes no mention of a refinement of the microstructure.
This paper states that a hydrogenation of Ti-6Al-4V alloy as an (.alpha.+.beta.) titanium alloy improves the hot workability through a lowering of the .beta. transformation point, and provides a fine microstructure. The hot working is conducted by forging at a reduction of 60% or less, and the forging is conducted in a slow speed ram motion system at a ram speed of 1.27 .times.10.sup.-3 or less. Namely, this working is not a practical working such that a strong working can be conducted by hot rolling, etc.
This paper states that a hydrogenation of Ti-6Al-4V alloy as an (.alpha.+.beta.) titanium alloy followed by hot rolling improves the anisotropy of tensile properties. In this process, however, a hydrogenated plate is homogenized at 990.degree. C. for 2 hrs, and a 50% rolling at 730.degree. C. is conducted in several passes of a 10% reduction of each pass with a homogenization treatment of 10 minutes after each reduction, which renders this process unsuitable for practical use.
This patent discloses a method of refining the microstructure of cast titanium alloy articles, which method comprises heating a cast article at 780.degree. to 1020.degree. C. in a hydrogen-containing atmosphere to hydrogenate the cast article, cooling the hydrogenated cast article to room temperature at a controlled rate of 5.degree. to 40.degree. C./min, and heating the cooled hydrogenated cast article in vacuum at 650.degree. to 750.degree. C. for dehydrogenation.
This patent discloses a method of refining the microstructure of prealloyed titanium alloy powder compacts, which method comprises heating a compacted article in a hydrogen-containing atmosphere at 780.degree. to 1020.degree. C. for hydrogenation, cooling the hydrogenated compacted article to room temperature at a rate of 5.degree. to 40.degree. C., and heating the cooled hydrogenated compacted article in vacuum at 650.degree. to 750.degree. C. for dehydrogenation.
The method disclosed in this paper comprises hydrogenating Ti-6Al-4V alloy as an (.alpha.+.beta.) titanium alloy, heating the hydrogenated alloy at 870.degree. C., subjecting the heated alloy to eutectoid transformation at 540.degree. to 700.degree. C., and dehydrogenating the transformed alloy at 650.degree. to 760.degree. C. to obtain a fine equiaxed microstructure.
Nevertheless, the above-described prior arts do not provide a sufficiently fine equiaxed microstructure, i.e., are unsatisfactory when attempting to stably prepare titanium and titanium alloys having a high strength, fatigue properties, and workability, etc., on a commercial scale.